April 15-17

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Author: Jason Parisi
Requested Type: Pre-Selected Invited
Submitted: 2019-02-20 11:59:20

Co-authors: F.I.Parra, C.M.Roach, M.Barnes, C.Giroud, P.G.Ivanov, N.Aiba

Contact Info:
University of Oxford
Merton College, Merton Street
Oxford, OX14JD,   00000
United Kingdom

Abstract Text:
Electron temperature gradient physics is shown to dominate the linear gyrokinetic spectrum in JET-ILW pedestals at all scales where kyρe>LTe/R, where LTe/R<sqrt(meTe/miTi)«1 due to steep gradient profiles. Local linear gyrokinetic simulations of four JET pedestal shots demonstrate that with the exception of KBMs, all other microinstabilities are driven primarily by the electron temperature gradient - many of these instabilities exist at turbulent transport-relevant scales. ITG is subdominant, and both KBMs and ITG are shown to be suppressed by flow shear. The four pedestal shots are 82550 (highly fueled, Ip=2.5 MA, H98=0.72, βΝ=1.1), 92167 (high D injection, Ip=1.4 MA, H98=0.93, βΝ=2.2), 92168 (low D injection, Ip=1.4 MA, H98=1.0, βN=2.6), 91274 (C2D4 injection, Ip=1.4 MA, H98=1.0, βΝ=2.5). Collisionality is typically νeea/vti«1. Electron temperature gradients are particularly large, causing ETG to be driven at perpendicular scales R/LTe>sqrt(miTi/meTe) larger than in the core. This makes ETG the fastest growing modes at kyρe~LTe/R«1. Results show ETG dominating at kyρi>0.5, which is mainly toroidal at low ky and slab at larger ky. The toroidal ETG mode has a small k|| and a sufficiently large radial wavenumber that FLR effects become important; that is, kyρi~1 but kxρe~1. The radial wavenumber for toroidal ETG is forced to be very large by the steep R/LTe profiles. While ions are unimportant for toroidal ETG due to kperpρe≫1, at larger kyρi the fastest growing mode switches to a slab ETG mode, where ions are no longer damped by FLR effects. Both types of ETG modes are surprisingly well-described by simple local dispersion relations. While simulations have been limited to JET-ILW pedestals, we expect the argument that R/LTe≫1 causes a shift of the linear ETG and ITG spectrum to smaller ky scales by a factor of R/LTe to hold more generally. The nonlinear role of these ETG-driven modes is still under investigation.

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